This invention relates to a hydrofoil boat of which hull and whole-submerged hydrofoil generate a dynamic lift for navigation.
Conventionally, a well-known hydrofoil boat has a deep front bottom and a shallow rear bottom that are distinguished from each other by forming a single step in the bottom, wherein a strut projects downward from the rear of front bottom so as to support a hydrofoil. For example, it is disclosed in JP,9-207872,A. In this disclosed art, the hydrofoil is a whole-submersed hydrofoil arranged deeper than the rear end of front bottom. While a boat is hydroplaning, rear ends of the respective front and rear bottoms serve as skid surfaces touching the water surface, and the submerged hydrofoil generates a dynamic lift for supporting, thereby reducing drag. This dynamic lift is controlled to be fixed for acquiring stability of navigation. High hydroplaning capacity can be acquired by enlarging a rate of dynamic lift loaded on the hydrofoil (a degree of mass supported by the dynamic lift of hydrofoil in the whole mass of a boat, which is expressed with a hydrofoil-dynamic-lift/boat-mass.).
However, the dynamic lift of the hydrofoil must be decided considering its relation to a centroid of the boat in connection with navigation stability, wherein the problem about the conventional hydrofoil boat having the single-stepped bottom exists. This problem will be explained in accordance with FIGS. 1 and 2. FIG. 1 illustrates schematic side views of a hydrofoil boat 1xe2x80x2 showing variation of pressure distribution and trim moment situation caused by the difference of the rate of dynamic lift loaded on the hydrofoil. FIG. 1(a) is the view when the rate of dynamic lift loaded on the hydrofoil is small, FIG. 1(b) is the view when it is fair, and FIG. 1(c) is the view when it is large. FIG. 2(a) is a schematic side view of conventional hydrofoil boat 1xe2x80x2 with a single-stepped bottom, showing an optimal centroid Gi when the hydrofoil is put at its navigation position, and FIG. (b) is a schematic view of the same, showing optimal centroid position Gi when there is no hydrofoil.
In hydrofoil boat 1xe2x80x2 shown in FIGS. 1 and 2, which has the single step as mentioned above formed in the bottom thereof, a hydrofoil 21 is arranged ahead of a step 1xe2x80x2a because it is stored so as to fit step 1xe2x80x2a. Therefore, a dynamic lift is generated at this position. This dynamic lift is desired to be close to the boat-centroid as much as possible. The reason is that, if the dynamic-lift generating position is far forward from the boat-centroid, porpoising occurs easily, and if it is far rearward, yawing occurs easily.
However, in the conventional hydrofoil boat having the single-stepped bottom exists such a problem that the centroid moves rearward with the increase of hydrofoil-dynamic-lift so as to raise a trim angle extremely. If hydrofoil boat 1xe2x80x2 having the single-stepped bottom shown in FIG. 1 is put in a state where a hydrofoil-dynamic-lift F (correctly, a rate of dynamic lift loaded on the hydrofoil) is small as shown in FIG. 1(a), a boat-centroid G is arranged just ahead of step 1xe2x80x2a so as to approach the position of hydrofoil-dynamic-lift. The front bottom section and the rear bottom section receive pressure from the water level surface so as to generate a pressure-distribution Pf in the front bottom section, and a pressure-distribution Pr in the rear bottom section. This pressure interacts as a force for supporting the boat. In the state of (a), a great distance between both pressure-distributions Pf and Pr is assured, and a trim moment Tm which interacts for balancing the boat in the length thereof is assured greatly.
However, as the hydrofoil-dynamic-lift is enlarged as shown in FIG. 1(b) and (c), pressure-distribution Pf in the front bottom section moves backward toward step 1xe2x80x2a while pressure-distribution Pr in the rear bottom section hardly moves because it is in a stem, whereby the trim angle is increased. In the state of (c), pressure distribution Pf in the front bottom section almost laps with the dynamic-lift generating position so that trim moment Tm becomes extremely small, whereby the boat remains tilting backward, i.e., porpoising.
Moreover, FIG. 2(a) shows hydrofoil boat 1xe2x80x2 provided with the hydrofoil, and FIG. 2(b) shows it having no hydrofoil. However, the state of (a) may be transposed to hydrofoil boat 1xe2x80x2 having hydrofoil 21 set at a navigating position, and the state of (b) to it having hydrofoil 21 stored. As shown in FIG. 2(a), when hydrofoil 21 is set to the navigating position, an optimal centroid Gi is arranged ahead of step 1xe2x80x2a so as to be close to hydrofoil 21, i.e., the dynamic-lift generating position. However, since there is no support by the hydrofoil-dynamic-lift when hydrofoil 21 is stored, pressure-distribution Pf in the front bottom section comes ahead and the boat tilts forward so that optimal centroid Gi moves rearward, as indirectly shown in FIG. 1(a). In the state of (b), it is located more rearward than step 1xe2x80x2a. When there are many loads, optimal centroid Gi when the hydrofoil being stored comes further rearward. However, if a boat-centroid G is established according to such optimal centroid Gi when the hydrofoil being stored, actual boat-centroid G will separate from optimal centroid Gi back increasingly during navigation, and the above faults will be brought about.
In order to avoid such porpoising in navigation, the hydrofoil-dynamic-lift must be set small. Consequently, the boat has not been able to acquire sufficient hydroplaning capacity.
A hydrofoil boat having a whole-submerged hydrofoil according the present invention comprises a couple of front and rear steps formed in a bottom thereof, and comprises a strut supporting a hydrofoil arranged between the pair of steps. In this way, the fluctuation of a trim angle accompanying the variation of hydrofoil-dynamic-lift is inhibited so as to prevent a gap between a boat-centroid and a dynamic-lift generating position which is as much as to generate porpoising. Therefore, high hydrofoil-dynamic-lift can be established, whereby the boat can acquire sufficient hydroplaning capacity. Moreover, even if the centroid is fluctuated by variation of the loading weight etc., the fluctuation is so small as to maintain the centroid adjacent to the dynamic-lift generating position, whereby the trim angle is not fluctuated greatly. Thus, high hydrofoil-dynamic-lift can be established in the same way, thereby effecting enough reduction of drag.
Moreover, in the hydrofoil boat of the present invention, the hydrofoil is rotated forward and rearward so that the position thereof may be changed between a storage position and a navigation position. The hydrofoil can be fixed in the navigating position corresponding to the variation of hydrofoil-dynamic-lift. Accordingly, as mentioned above, the hydrofoil-dynamic-lift can be kept constant so as to acquire stability in navigation, as well as it can be set to be high as mentioned above.
Furthermore, the hydrofoil boast of the present invention can adjust the hydrofoil-dynamic-lift and a tilt angle of a propulsor or a flap disposed on the rear of hull, corresponding to draft or the trim angle when the boat berths. That is, even if the mass of boat changes by variation of loading weight, the hydrofoil-dynamic-lift can be accommodated and established so as to hold the rate of dynamic lift loaded on the hydrofoil constant. Therefore, the boat can acquire the best reduction of drag corresponding to the boat-mass that varies at any time. Further, it can acquire an optimal propelling force corresponding to the fluctuation of trim and draft by adjusting the set angle of the propulsor or the flap. In this way, the present invention serves as a hydrofoil boat having high hydroplaning capacity.